Mono crystalline diamond cutting tool for ultra precision machining

ABSTRACT

A mono crystalline diamond cutting tool is provided which can perform ultra precision machining on a crystalline material or a hard and brittle material with good swarf discharge and reduced cutting resistance to improve a precision of a cut surface, and has less wear or microchipping of a cutting edge and thus achieves long life. A tip having a cutting edge ridge in a rounded shape at a front end is provided, and a portion of the cutting edge ridge serving at least as a cutting edge is formed to have constant roundness by intersecting a first conical surface as a rake face with a second conical surface as a flank. The cutting edge ridge is rounded with a radius of less than 100 nm, the first conical surface has a width of 1 to 5 μm, and a swarf release face substantially perpendicular to a cutting direction is provided in a portion on a side of the first conical surface opposite a line of the cutting edge ridge. An intersection of the first conical surface and the swarf release face has a rounded face with a radius of 0.1 to 1.0 μm. The first conical surface has a negative rake angle of 15° to 50°.

TECHNICAL FIELD

The present invention relates to a mono crystalline diamond cutting toolto perform ultra precision machining on a crystalline material such asSi, Ge, or CaF₂, and on a hard and brittle material used for such as acemented carbide, glass, or a mold base material.

BACKGROUND ART

Recently, to cope with rapid diffusion of optoelectronics technologyinto such as digital consumer electronics and needs for ahigh-precision, highly functional product, a crystalline material suchas Si, Ge, or CaF₂, and a hard and brittle material such as a cementedcarbide mold or glass are used, and an ultra precision diamond cuttingtool is used to cut these materials with high precision. An example ofthe cutting tool is a diamond cutting tool using a mono crystallinediamond tip as a cutting edge. As a specific example of the diamond tip,there is a diamond tip having a conical rake face to finish with goodshape accuracy and surface roughness when turning a brittle material tohave a curved surface (see for example Japanese Patent Laying-Open No.63-237803 (Patent Document 1)).

Further, as a tool having a shape similar to that of the above-mentioneddiamond tip, a mono crystalline cutting tool is provided in which a noseportion of the cutting tool is rounded to have a rake face with anegative rake angle, and the rake face is formed as a portion of aconical surface of a right circular cone (see for example JapanesePatent Laying-Open No. 64-64702 (Patent Document 2)).

Furthermore, a diamond cutting tool is provided in which an edge portionmade of mono crystalline diamond is fixed to a tool body such that arake face of the edge portion has a rake angle set negative in the rangefrom −25° to −60° as shown in FIGS. 5A to 5C to cut a ductile anddifficult-to-machine material such as high silicon aluminium orNi-resist cast iron (see for example Japanese Patent Laying-Open No.11-347807 (Patent Document 3)). In addition, as a method to performultra precision machining on a crystalline material such as a ZnSe lens,a method is provided by which a single cutting tool performs cuttingfrom rough work to finish work using a mono crystalline diamond cuttingtool having a rake angle of −20° to 20°, a clearance angle of 5° to 10°,and a cutting edge chamfered in a width of 0.5 to 2 μm to prevent theedge from getting chipped, as shown in FIGS. 6A and 6B (see for exampleJapanese Patent Laying-Open No. 10-43903 (Patent Document 4)).

Patent Document 1: Japanese Patent Laying-Open No. 63-237803

Patent Document 2: Japanese Patent Laying-Open No. 64-64702

Patent Document 3: Japanese Patent Laying-Open No. 11-347807

Patent Document 4: Japanese Patent Laying-Open No. 10-43903

DISCLOSURE OF THE INVENTION

Problems to be Solved by the Invention

When performing ultra precision machining on a difficult-to-machinematerial, a crystalline material, or a hard and brittle material using amono crystalline diamond cutting tool as in the above Patent Documents 3and 4 , the rake angle is set negative to prevent precision degradationin a cut surface and chipping of the edge. However, when the rake angleis set negative, the rake face is generally provided as a sloping faceas shown in FIG. 3, and there arise problems described below when such arake face is employed in a cutting tool having a cutting edge in arounded shape.

Firstly, even when the rake angle is set negative, an effective rakeangle varies depending on a position where the cutting edge works, andthe cutting edge on a rear end side has a significantly smaller negativerake angle than the cutting edge on a front end side, due to the reasondescribed below. FIG. 4 schematically shows the case where a diamondcutting tool is used in a two-axis controlled turning machine, which ismost commonly used, to cut a workpiece to have a spherical surface.Workpiece 11 is rotated around a rotation axis 12, and it is cut as adiamond cutting tool 1 is fed in X-axis and Z-axis directions. In thiscase, when diamond cutting tool 1 is on the rotation axis, a front endportion A of a cutting edge 5 works on the workpiece, and a slopingangle of a rake face serves as a rake angle on this occasion. However,when diamond cutting tool 1 is fed in the X-axis and Z-axis directionsand a portion B of cutting edge 5 comes to work on the workpiece, therake angle becomes smaller. As a result, an effective rake angle varies,causing variations in surface roughness of the workpiece. Further, theworking portion of cutting edge 5 also moves in a Y-axis direction(although not shown in FIG. 4, the Y-axis direction refers to adirection perpendicular to the plane of the drawing), and diamondcutting tool 1 should also be moved in the Y-axis direction. Althoughdiamond cutting tool 1 can be moved in the Y-axis direction when athree-axis controlled turning machine is used, it cannot be moved in thecase of using a two-axis controlled turning machine. Since a two-axiscontrolled turning machine is often used in ultra precision machining atpresent, this results in deterioration of cutting precision.

Secondly, there arises a problem caused by the cutting edge having anoval shape instead of a constantly rounded shape. When a workpiece iscut with such a cutting edge to have a spherical or aspherical surface,it is necessary to perform cutting once, calculate a correction valuebased on a cut shape and formulate a cutting program, and performoriginally desired cutting using the program, because the position ofcutting edge 5 in the Y-axis direction varies depending on the positionwhere cutting edge 5 works as described above. Since the above processinvolves a correction step, it requires much time and effort.

Thirdly, since the height of the cutting edge also varies depending onthe position where the rounded cutting edge works, stable cutting cannotbe provided, which may result in a shape error. This is because theposition of cutting edge 5 in the Y-axis direction varies, that is, acutting tool's edge height varies, depending on the position wherecutting edge 5 works as described above. In particular, a significantshape error is likely to occur when a workpiece is cut to have anaspherical surface.

These three problems described above could be solved to some extent byusing a tool having a cutting edge in a shape as described in PatentDocument 1 or 2. However, since a large portion of the rake face alongthe cutting edge has a negative angle in such a tool as in the tooldescribed in Patent Document 3 or 4, swarf is likely to accumulatebetween the rake face and a cut surface, deteriorating a precision ofthe cut surface. Specifically, swarf generated at the position where thecutting edge works flows over the rake face when it flows from the frontend side to the rear end side of the cutting edge, it is likely toaccumulate between the rake face and the cut surface, causing variationsin swarf discharge. Consequently, the cut surface does not have stablequality, and thus the precision of the cut surface is deteriorated.

Further, to form a cutting edge in a shape described in each patentdocument, the amount of diamond being cut is increased, leading to anincrease in manufacturing cost. This is because forming a rake facerequires an increased amount of diamond being cut, and takes time andeffort.

Furthermore, in the case where cutting is performed on a material asdescribed above, wear or microchipping of the cutting edge occurs evenwith a diamond tip and a diamond cutting tool as described in eachpatent document, which may shorten tool life.

In view of these facts, one object of the present invention is toprovide a mono crystalline diamond cutting tool capable of performingultra precision machining on a crystalline material or a hard andbrittle material with high precision without causing a shape error inthe cut material, and having less wear or microchipping of a cuttingedge and thus achieving long life.

Means for Solving the Problems

A mono crystalline diamond cutting tool for ultra precision machining ofthe present invention has a first characteristic that it is a diamondcutting tool provided with a mono crystalline diamond tip having acutting edge ridge in a rounded shape at a front end, and that a portionof the cutting edge ridge serving at least as a cutting edge is formedto have constant roundness by intersecting a first conical surface as arake face with a second conical surface as a flank, the cutting edgeridge is rounded with a radius of less than 100 nm, the first conicalsurface has a width of 1 to 5 μm, and a swarf release face substantiallyperpendicular to a cutting direction is provided in a portion on a sideof the first conical surface opposite a line of the cutting edge ridge.

The tool has a second characteristic that an intersection of the firstconical surface and the swarf release face has a rounded face havingpredetermined roundness.

The tool has a third characteristic that the rounded face has a radiusof 0.1 to 1.0 μm.

The tool has a fourth characteristic that the first conical surface hasa negative rake angle of 15° to 50°.

The tool has a fifth characteristic that there is no lattice defect in adiamond crystal within a range of 100 μm from a front end of the cuttingedge.

Effects of the Invention

The mono crystalline diamond cutting tool of the present invention canperform ultra precision machining on a crystalline material or a hardand brittle material with good swarf discharge and reduced cuttingresistance, improving a precision of a cut surface. Further, it canperform cutting with high precision without causing a shape error in thecut material, and its life can be improved since it has less wear ormicrochipping of the cutting edge.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1A is a plan view showing a mono crystalline diamond tool of thepresent invention.

FIG. 1B is a front view showing the mono crystalline diamond tool of thepresent invention.

FIG. 1C is a fragmentary enlarged front view showing a portion in thevicinity of a cutting edge of the mono crystalline diamond tool of thepresent invention.

FIG. 2 is a perspective view showing an example of a diamond tip of themono crystalline diamond tool of the present invention.

FIG. 3 is a perspective view showing an example of a diamond tip of aconventional mono crystalline diamond tool.

FIG. 4 illustrates a state during cutting with a two-axis controlledturning machine.

FIG. 5A is a plan view showing another example of the conventional monocrystalline diamond tool.

FIG. 5B is a front view showing the another example of the conventionalmono crystalline diamond tool.

FIG. 5C is a fragmentary enlarged front view showing a portion in thevicinity of a cutting edge in the another example of the conventionalmono crystalline diamond tool.

FIG. 6A is a plan view showing still another example of the conventionalmono crystalline diamond tool.

FIG. 6B is a front view showing the still another example of theconventional mono crystalline diamond tool.

FIG. 7A is a plan view showing another example of the diamond tip of theconventional mono crystalline diamond tool.

FIG. 7B is a front view showing the another example of the diamond tipof the conventional mono crystalline diamond tool.

DESCRIPTION OF THE REFERENCE SIGNS

1 diamond cutting tool

2 tip

3 rake face

4 flank

5 cutting edge

6 swarf release face

7 rounded face

8 tool body

BEST MODES FOR CARRYING OUT THE INVENTION

FIGS. 1A to 1C show a mono crystalline diamond cutting tool as anexample of a mono crystalline diamond cutting tool of the presentinvention, and FIG. 2 shows a fragmentary enlarged perspective view of atip thereof. In a diamond cutting tool 1, a tip 2 made of monocrystalline diamond is fixed at a front end portion of a tool body 8made of such as a cemented carbide, by means of brazing or the like. Tip2 has a cutting edge 5 in a rounded shape at a front end, and cuttingedge 5 is formed by intersecting a rake face 3 with a flank 4. A portionof rake face 3 serving at least as a cutting edge is formed of a firstconical surface, and the portion has a width of 1 to 5 μm and a negativerake angle of 15° to 50°. A portion of flank 4 serving at least as acutting edge is formed of a second conical surface. Both of the faceshave planes on a rear end side continuing from the respective conicalsurfaces, and a linear ridge line not serving as a cutting edge isformed at an intersection of these planes.

With this shape, an effective rake angle does not vary even when theposition where the rounded cutting edge works varies, providing stablecut surface roughness. Consequently, a cut surface having high precisioncan be obtained, and control over the position of the cutting edge canbe facilitated. Further, since the rake face has a conical surface, thecutting edge can have constant roundness.

It is to be noted that a width of rake face 3 in the present inventionrefers to a distance L between a point X where rake face 3 and flank 4intersect and a point Y where rake face 3 and a swarf release face 6intersect, as shown in FIG. 1C.

The ridge line of cutting edge 5 is rounded with a radius of less than100 nm, providing a sharp cutting edge. This shape allows ultraprecision machining with a sharp edge.

Swarf release face 6 is formed on the rear end side of the conicalportion of rake face 3 (on the side opposite to the side on whichcutting edge 5 is formed), as a face substantially perpendicular to thecutting direction. A rounded face 7 is formed at an intersection of rakeface 3 and swarf release face 6. Thereby, in conjunction with an effectof the rake face having a conical shape with a minute width, swarfdischarge is improved, causing no adverse effect such as reduction in aprecision of a cut surface. Preferably, rounded face 7 has roundnesswith a radius P of 0.1 to 1.0 μm to improve swarf discharge and reducecutting resistance.

To form cutting edge 5, the mono crystalline diamond is provided to haveno lattice defect in a diamond crystal within a range of 100 μm from thefront end, and thus there is no lattice defect in the vicinity ofcutting edge 5. Thereby, wear or microchipping less occurs even whensharp cutting edge 5 is provided as described above, extending toollife.

First Example

As a mono crystalline diamond tool of the present invention, a diamondcutting tool shown in FIGS. 1A to 1C (hereinafter will be referred to asan example of the present invention) was fabricated. As comparativeexamples, a conventional mono crystalline diamond cutting tool shown inFIGS. 5A to 5C (hereinafter will be referred to as a first comparativeexample) and a mono crystalline diamond cutting tool having a cuttingedge shown in FIGS. 7A and 7B (hereinafter will be referred to as asecond comparative example) were fabricated. These tools were used tocut mono crystalline silicon for performance comparison. In the exampleof the present invention, a width L of a rake face was set at 1.2 μm, arake angle α was set as a negative angle of 15°, a cutting edge ridgeline had roundness with a radius r of 60 nm, and rounded face 7 had aradius P of 0.3 μm. In the first comparative example, width L of a rakeface was set at 100 μm, rake angle α was set as a negative angle of 25°,an intersection of rake face 3 and swarf release face 6 had anon-rounded edge, and a cutting edge ridge line had roundness withradius r of 100 nm. In the second comparative example, rake angle α wasset at 0°, and a cutting edge ridge line had roundness with radius r of100 nm. In each diamond cutting tool, a radius R of the cutting edgeridge was set at 1.2 mm.

Each of these diamond cutting tools was attached to a two-axiscontrolled turning machine to cut the mono crystalline silicon to have aspherical surface by means of CNC two-axis control technique. Wetcutting was performed with the number of workpiece spindle revolutionsof 2000 rpm, a tool feed rate of 0.00175 mm/revolution, and a depth of0.0015 mm. To measure cutting resistance during the cutting, a vibrationacceleration sensor was attached on the rear end side of the diamondcutting tool to measure vibration acceleration.

As a result of performing the cutting with the tools and under thecondition as described above, surface roughness Ra obtained in aninitial stage of the cutting was 0.0067 μm in the example of the presentinvention. The surface roughness was gradually deteriorated as thecutting was repeatedly performed, and in the end, 60 workpieces were cutwith allowable surface roughness. Further, a PV (peak and valley) valueof 0.042 μm, an rms (root mean square) value of 0.009 μm, and cuttingresistance during the cutting (vibration acceleration) of 0.05 G wereobtained.

In contrast, in the first comparative example, surface roughness Raobtained in an initial stage of the cutting was 0.0085 μm. The surfaceroughness was gradually deteriorated as the cutting was repeatedlyperformed, and in the end, 25 workpieces were cut with allowable surfaceroughness. Consequently, the tool life was less than half that in theexample of the present invention. Further, a PV value of 0.047 μm, anrms value of 0.010 μm, and cutting resistance during the cutting(vibration acceleration) of 0.08 G were obtained, all of which werehigher than the values obtained in the example of the present invention.

In the second comparative example, surface roughness Ra obtained in aninitial stage of the cutting was 0.0138 μm, almost double that of theexample of the present invention. Since chipping occurred during thecutting, only two workpieces were cut. Further, a PV value of 0.081 μm,an rms value of 0.018 μm, and cutting resistance during the cutting(vibration acceleration) of 0.2 G were obtained, all of which weresignificantly higher than the values obtained in the example of thepresent invention.

As have been described above, it has been found that the monocrystalline diamond tool of the present invention has good swarfdischarge, improving the precision of the cut surface. It has also beenfound that the tool can perform cutting with high precision withoutcausing a shape error in the cut material, and its life is improvedsince it has less wear or microchipping of the cutting edge.

INDUSTRIAL APPLICABILITY

The mono crystalline diamond tool of the present invention is applicableto a cutting tool for performing ultra precision machining on acrystalline material, a hard and brittle material, adifficult-to-machine material, or the like.

1. A diamond cutting tool provided with a mono crystalline diamond tiphaving a cutting edge ridge in a rounded shape at a front end, wherein aportion of said cutting edge ridge serving at least as a cutting edge isformed to have constant roundness by intersecting a first conicalsurface as a rake face with a second conical surface as a flank, saidcutting edge ridge is rounded with a radius of less than 100 nm, saidfirst conical surface has a width of 1 to 5 μm, and a swarf release facesubstantially perpendicular to a cutting direction is provided in aportion on a side of said first conical surface opposite a line of saidcutting edge ridge, and a rounded face joins said first conical surfaceand said swarf release face.
 2. The mono crystalline diamond toolaccording to claim 1, wherein said rounded face has a radius of 0.1 to1.0 μm.